A transformer having a structure in which a conductive wire is wound around a core (also referred to as a magnetic core, an iron core, and the like) is frequently used in a power converter of a voltage, a current, or the like. In order to convert a large power by using the transformer, a large core and a thick conductive wire are required, and thus, in order to reduce the size and the weight of the power converter, first, a reduction in the size of the transformer is strongly required.
In a case where the conductive wire wound around the core is thinned in order to reduce the size and the weight of the transformer, the resistance of the conductive wire increases, the temperature of the core increases due to Joule heat thereof, and the core causes magnetic saturation, and thus, a power conversion function as the transformer is lost. In addition, in the case of simply reducing the size of the core without considering an increase in the temperature, a magnetic flux density in the core increases, and thus, naturally, the magnetic saturation easily occurs. Therefore, in order to reduce the size of the transformer, it is necessary to prevent the core from reaching the magnetic saturation by suppressing an increase in the temperature of the core or a winding portion (the coil).
In PTL 1, an example of a transformer including a protrusion of a center portion, which becomes a middle foot by being wound around with a conductive wire, and a ferrite core formed of four leg portions branching off from the outer circumference of the center portion, is disclosed (refer to FIG. 9 or the like). Then, improvement in the heat radiation properties of the ferrite core is described as an effect of including four leg portions.